The formed wire box spring assembly currently in use consists of a generally rectangular frame having a pair of side rails and a pair of end rails, a plurality of cross rails extending between the side rails, a border wire running along the perimeter of the frame as defined by the side and end rails, a plurality of longitudinal and transverse main springs each terminating at and being attached to the border wire, and several support springs located within the borders of the frame. The springs of this assembly are of the formed wire type, being composed of a straight wire bent at both ends so that the bent portion of the wire comprises a series of bars arranged in a zigzag shape and capable of resisting loads. The main springs are attached to the border wire and the support springs are attached to the main springs by metal clips each of which encircles two side-by-side portions of wire and holds them together. Because there is limited friction between metal wire and metal clip, the clips tend to slide on the wire. When a spring is imperfectly formed or mounted, it may even tend to slide the clip out of position. This movement in turn enables the springs to move out of position which adversely affects the functional performance of the assembly. It is an object of the present invention, therefore, to provide an improved box spring assembly in which the clips are mounted on plastic coated wires which prevent the clips from slipping thereby insuring that each spring will be secured in its intended position in which it will act and react in the way in which it was intended.